The present invention relates to optical amplifiers and in particular to wide bandwidth optical amplifiers. The present invention is particularly applicable to Raman amplifiers.
Raman amplification is considered to be particularly suitable for use in wideband and ultra wideband optical amplifiers, for example used in wavelength division multiplexing transmission systems. The reason for this is that, unlike Erbium doped fibre amplifiers, gain can be provided at a predetermined range of wavelengths by choosing a suitable pump wavelength. In addition, Raman amplification can be distributed along a transmission fibre, giving an improved optical signal-to-noise ratio, lower signal launch power and therefore longer transmission distances without requiring further amplification and/or repetition.
However one drawback is that for a Raman amplifier using a single pump wavelength only, the gain profile of the amplifier is far from flat. Typically the gain profile will be a curve with a peak at the wavelength chosen for maximum amplification. Often it is desirable for the amplifier to operate adequately over a range of wavelengths and so in order to improve the flatness of the amplifier gain profile over a wide range of wavelengths, several pump sources may be used.
FIG. 1 shows a prior art wideband Raman optical amplifier. In FIG. 1, a signal to be amplified is input to fibre 2. In addition, pump radiation is provided to fibre 2 through the signal and pump coupler 4. The resulting amplified signal is output on fibre 6. N different pump sources (generally indicated 8) are provided and each pump source produces pump radiation of a different wavelength from the other pump sources i.e. pump radiation of wavelengths λ1, λ2, λ3 . . . λN are provided.
All the sources of pump radiation are fed to a multiplexer 10 which multiplexes the pump sources and transmits the resulting wideband wavelength pump radiation to the signal and pump coupler 4. In this way, signals travelling on the fibre connected to the signal coupler 4 may be amplified.
This solution is based on the assumption that the Raman gain which results from these multiple pumps is close to the superposition of the Raman gains which would result from the use of each of the pump sources individually. Therefore, by choosing appropriate pump wavelengths, powers and taking into account interpump amplification a reasonably flat gain profile can be achieved over a predetermined wavelength band.